The present invention relates to a process for removing selenium from a fluid and, in particular, to a process for removing selenium from a waste water stream.
There is increasing concern over the hazards posed by the rising levels of pollutants within the world's water supplies. Pollutants include, but are not limited to, acenapthene, acrolein, acrylonitrile, aldrin, antimony, arsenic, asbestos, benzene, benzidine, beryllium, cadmium, carbon tetrachloride, chloralkyl ethers, chlordane, chlorinated benzenes, chlorinated ethanes, chlorinated, naphthalenes, chlorinated phenols, chloroform, chlorophenol, chromium, copper, cyanide, dichlorobenzenes, dichlorobenzidines, dichloroethylene, dichloropropane, dichloropropene, dieldrin, dimethyl phenol, dinitrotoluene, dioxin, endosulfan, ethylbenzene, fluoranthene, haloethers, halomethanes, heptachlor, hexachlorobenzene, hexachlorobutadiene, hexachlorocyclohexane, hexachlorocyclopentadiene, hexachlorinated ethanes, isophorone, lead, mercury, naphthalene, nickel, nitrobenzene, nitrophenols, nitrosamines, pentachlorinated ethanes, pentachlorophenol, phenol, phthalate esters, polynuclear aromatic hydrocarbons, selenium, silver, tetrachlorinated ethanes, tetrachlorinated benzene, tetrachloroethylene, tetrachlorophenol, thallium, toluene, toxaphene, trichlorinated ethanes, trichlorobenzene, trichloroethylene, trichlorophenol, vinyl chloride, and zinc. A more extensive list of pollutants is set forth on pages 9-10 of Quality Criteria for Water 1986, EPA Publication 440/5-86-001, also available from the U.S. Department of Commerce, National Technical Information Service as publication PB87-226759, the publication being incorporated in its entirety by this reference.
Another criteria used in describing waste water streams is the chemical oxygen demand (COD) of the stream. (Tests for determining the COD of an aqueous medium are well know to those skilled in the art and include the Open Reflux Method, the Closed Reflux Titrimetric Method, and the Closed Reflux Colorimetric Method.) Although waste water streams can have a COD as low as about 25 mg oxygen/l, waste water streams generally have a COD of at least about 50 mg oxygen/l, and more typically at least about 100 mg oxygen/l. However, it is not uncommon for waste water streams to have a COD greater than about 500 mg oxygen/l or even greater than 1,000 mg oxygen/l. In fact, quite often, waste water streams have a COD greater than about 2,500 mg oxygen/l and, indeed, greater than about 5,000 mg oxygen/l.
Regarding selenium, the toxicity of selenium is evidenced by the following excerpt from The Merck Index, 10th Edition, Windholz et al. Editors, Merck & Co., Inc., Rahway, NJ (1983): "Occupational exposure has caused pallor, nervousness, depression, garlic odor of breath and sweat, G.I. disturbances, dermatitis. Liver injury has been produced in [experimental] animals."
Techniques for removing selenium from waste water streams are known. See, for example, PCT International Application WO 84/03692, the publication being incorporated herein in its entirety by this reference. Nevertheless, there is a need to increase the efficiency of these selenium removal processes and to provide techniques for removing selenium from waste waters substantially unaffected by present selenium removal procedures.